Growing ultrapure or doped single crystals having a low defect density and uniform residual impurity or dopant levels, respectively, over the crystal length as well as radially has always been difficult to achieve. The type of crystal and the method by which they are grown can contribute a great deal to the residual impurities or dopant distribution. The crystal defect density is also affected by the crystal growth rate, crystal size, and the temperature gradients in the furnace. Some post growth annealing is usually required to suppress one type of crystal defect or to generate another type of crystal defect as required by the particular use of the crystal.
Crystals comprised of a single element, two elements (binary compounds) or three elements (ternary compounds) have been the subject of numerous experiments and have found a wide range of applications. Silicon and Group III-V compounds, Gallium-Arsenide (GaAs) in particular, are used for single device or integrated circuit fabrication. The alkali halide crystals have found numerous applications in the area of nuclear physics, medical imaging, and high energy physics.
Most of the high energy facilities today have a detector chamber consisting of approximately 10,000 crystals of CsI having an average size of 4.times.4.times.12 inches. The high resolution within a wide energy range makes it possible to analyze various particle-particle interactions. Sodium Iodide ((NaI) (Tl)) is frequently used in the area of medical imaging. Plates of various shapes and sizes are used to image particular sections of the human body. The optimal goal of using a single plate capable of scanning the entire human body in a single event has yet to be achieved. The present limitation is mainly due to the state of the art of the crystal growth process.
To produce alkali halide crystals which will be at least double in length and will have a uniform residual impurity/dopant distribution a new method has been proposed. Up to now, alkali halide crystals are produced exclusively in a batch method using either the Bridgman-Stockbarger or Kyropolos methods. Part of the difficulty in using continuous growth methods for alkali halide crystals is their water soluble characteristics, and their tendency to absorb and retain water during the liquefication and crystallization process.
This application deals with an improved method for growing alkali halide crystals of vastly improved quality for experimentation and production.
Alkali halide crystals are usually grown by the Bridgman-Stockbarger method of crystal growth. The Bridgman-Stockbarger method utilizes a heated chamber which is divided into a upper and lower heated area. These areas are separated by a baffle to allow independent control of the temperature in the upper and lower portions of the chamber. The upper section of the chamber is heated to a temperature of approximately 50.degree.-150.degree. C. above the melting point of the crystal and the lower section is maintained at a temperature slightly below the melting point of the crystal. Thus, the flow of heat is from the upper section of the chamber downwards through the melt. This will cause crystallization to begin at the lower section and proceed upwards through the melt as the crystal is lowered from the upper portion of the chamber into the lower portion. During the growth of the crystal, the temperature of the melt is higher in the melt than it is at the interface to the growing crystal or at the lower surface.
During the melt down of the charging material, the crucible is positioned in the upper section of the chamber. All air is evacuated, then an inert gas such as nitrogen is added at a pressure of 1 to 5 PSI above the ambient pressure in the room. The inert gas will not react with the alkali halide and will also serve to suppress evaporation of the melt itself. This slight over pressurization with an inert gas also reduces impurities by preventing any outside contaminants from leaking into the crystallization furnace. Occasionally, a small amount of certain other gases, called scavengers, are added to the chamber in order to react with and draw out other impurities which are known to exist within the melt.
After the alkali halide is completely melted and a desirable temperature is established in the melt, the crucible is slowly lowered in the chamber past the baffle into the cooler lower section. The alkali halide crystallizes from the bottom of the melt on up as the temperature in the melt drops below the melting point in the vicinity of the baffle. The melt-to-crystal interface remains nearly stationary in horizontal proximity to the baffle with a slow downward movement of the crucible.
The temperature gradient in the melt along with the interface is maintained in a fairly steady state by close control of the power applied to the heaters in both the upper and lower portions of the chamber. This also results in a quiet melt, which is devoid of turbulence. This allows all impurities or dopants to diffuse uniformly throughout the melt, and allows the lattice structure of the crystal to form without defects.
Prior work in the area includes the work of Hammond et al. in U.S. Pat. No. 4,030,965 which addresses the particular problem of bubble formation within the melt and the resulting crystal.
Swinehardt in U.S. Pat. No. 4,055,457 address metal impurities found in trace amounts in alkali halides grown with the Stockbarger and Kyropolos methods. Swinehardt concerned himself primarily with optical characteristics of potassium and sodium salts.
Swinehardt found quartz crucibles most suitable for crystallization and platinum completely unsuitable. Swinehardt also noted the shrinkage of charging material once melted. This particular problem stems from the fact that the packing density of the powder used to load the crucible is well below the packing density of the crystalline material. For example, charging material such as packed Sodium Iodide powder has a packing density of approximately 1.5-1.6 grams per cubic centimeter, and granular Sodium Iodine has a slightly higher packing density of approximately 1.75 grams per cubic centimeter. Crystalline Sodium Iodine has a packing density of approximately 3.67 grams per cubic centimeter. A crucible tightly packed with Sodium Iodine in either granular or powdered form will produce a crystal whose height is less than one half the height of the crucible.
Lorenzini et al in U.S. Pat. No. 4,036,595, Witter et al in U.S. Pat. No. 4,547,258, and Stock et al in U.S. Pat. No. 4,834,832 each suggested methods for continuous growing of crystalline material. Lorenzini and Witter both addressed their attention to the Czochralski and Kryopolos method of drawing a crystal upwards from a melt. Both Lorenzini and Witter support continuous flow processes. However, by their very nature, they lead to the creation of turbulence within the melt. This will lead to defects forming within the lattice structure of the crystal.
Lorenzini, Witter, and Stock all direct their attention to growing semiconductor crystals, particularly silicon, and it is not quite applicable for growth of alkali halides.
The invention of Stock, et al. involves dropping molten Silicon onto the top of an existing melt and drawing a crystal downwards through a cooling zone. The Stock method calls for continuous adding of granular silicon material and eliminates the use of a crucible.
In actual operation, alkali halides are extremely difficult to consistently dope with particular materials. This is because several dopants that are popularly used have segregation coefficients that are not equal to unity. Thus, if an over-all melt has a predetermined dopant concentration, as the crucible is lowered, the crystallized material will have a substantially lower concentration, with the concentration of dopant in the melt increasing correspondingly. This means that the dopant concentration can very drastically from one end of the crystal to the other. For example, Thallium distribution in Sodium Iodide or Cesium Iodide crystals from start to finish may vary by more than 20 times from the bottom to the top of the crystallization crucible. This leads to single crystal quality degradation due to the dopant segregation.